A US student hailed as a hero for protecting a friend from a knife-wielding classmate says that the attack was a "blur" in his mind but that when he realised he was bleeding he was thinking "will I survive or will I die?"

Brett Hurt, a sophomore at Franklin Regional High in Murrysville, Pennsylvania, suffered a stab wound in the back and a bruised lung during the attack Wednesday that left more than 20 students wounded. Three remained hospitalised in intensive care Thursday, officials at Forbes Regional Hospital said. At least two others remained hospitalised but were expected to be released within a few days.

Brett's friend, Gracey Evans, credits Brett with shielding her from the attacker. But Hurt told a hospital news conference that he remembers little of the attack.

"I remember messing around with Gracey, then getting hit in the back," Brett said. "That's when everything went straight into chaos."

"What was going through my mind?" he said in response to a media question. "Will I survive or will I die."

He said physicians told him he would be released soon from the hospital. He credited Gracey, a junior, with applying pressure to his wound at the scene, slowing the bleeding.

"She saved my life," Brett said.

'HEY, YOU'RE BLEEDING'

Gracey, 17, said she was with Brett when she heard a girl say, "'Hey, you're bleeding.' A random kid looks down and he's bleeding."

She saw a boy coming down the hallway stabbing students. Hurt stepped in between her and the boy with the knives, she said.

"In less than 30 seconds, I saw three people stabbed," she said.

"Hurt laid on his back. There was another kid (Greg Keener) who got stabbed pretty bad. I told him to sit up. I laid him down so his airway would be clear," she said.

She put pressure on Keener's wound until a medic arrived and took over.

"It wasn't all me," Gracey said, explaining that several students rallied with her to help their wounded classmates.

"Brett started screaming in pain, I went in and held his hand. ... I couldn't have done any of what I did if I didn't have help from the four people in the room with me. Helping me hold pressure on wound, calm down people and get them to stay awake. Everyone got held," she said.

DIDN'T KNOW ATTACKER

Brett said he had met the accused attacker, 16-year-old Alex Hribal, once or twice. But he said he does not really know him.

"He'll have to do the work to forgive himself," Brett said. "Everybody has more than one road to take in life. You chose what path to take. Everybody can make right or wrong decisions."

Brett's mother, Amanda Leonard, said she was thankful that her son's condition was improving. She said she has thanked Gracey.

"There is nothing in the world I can do for that girl that can thank her enough for what she has done," Amanda Hurt said.

SHY CHILD

Interviews with nearly two dozen students Wednesday evening at various vigils organised by churches yielded precious little background about Hribal, who was arraigned on charges of attempted homicide, aggravated assault and weapon possession.

Many students said they did not know him. Others, like sophomore Anissa Park, who knew Hribal from elementary school but hadn't talked with him for some time, invariably used the words "shy" and "quiet" to describe him. Some said he was involved in athletics, including street hockey, track and tennis.

Another student who knew Hribal, though he spoke on condition of anonymity, said he was stunned by the attack.

"I know him pretty well," the boy said, adding that Hribal's interests swung toward the usual for a teenage boy, including hockey and video games. "I've never seen any anger from him, ever. ... He never seemed like someone who would do anything violent. He never seems very upset or any of that."

Hribal's lawyer, Patrick Thomassey said he had spoken to them about an hour before the teen's arraignment.

"They did not foresee this coming. They expressed absolute horror," Thomassey said, adding that the family's thoughts were with the victims.

Thomassey said the teenager was not a loner nor was he aware of any instances of bullying that would have provoked the attack.

"He's scared," Thomassey said. "He's a young kid. He's 16, looking like he's 12. This is all still new to him."

Thomassey said Alex is a B-plus student from a stable home, describing his family as "like Ozzie and Harriet."

"They have dinner together every night," the attorney said, though he would not provide details of his discussions with his client.

Though she would not discuss the Franklin Regional case specifically, Mary Margaret Kerr, chair of administrative and policy studies and a professor of psychology in education and psychiatry at the University of Pittsburgh, said it can be difficult for schools to predict violent behavior in students.

She said standard psychological tests haven't been successful in predicting targeted violence in schools, adding that many school attackers had no histories of mental disorders.